I am thinking an easy way to check if your seeing is genuine sub arc sec or not might be to look visually through a small scope (6' or 8") at high power (or at a high powered image on a screen?) and see if you can detect any effect of the seeing ie is the image moving or blurring at all?... if you can then I would suggest it probably isn't sub arc sec seeing..?
Of course, this sort of situation is stuff of dreams
(has anyone seen it?) and I suspect would only occur regularly at the very best locations on Earth, probably high isolated mountain peaks for example?
I do remember a fantastic night at Mt Campbell (780m ASL), just outside Canberra, back in 2006, I (and the CAS President) were using both the 6" Starfire and 12" SCT and the steady moments showed some perfectly still views in the 6" but they only lasted for a few seconds at a time (which makes a big difference) but while also pretty good, the image never quite settled perfectly in the 12". So, the resolution of a 6" is about 0.75" so I am guessing I was getting several seconds at a time of sub arc sec seeing...? To enjoy hours of this (like at La Palma and Chilean sites) would be simply breathtaking
Mike